Paola Rizzi

Abstract:

Nanoporous gold (NPG) can be produced by dealloying [1], a process in which the less noble elements in an alloy are chemically or electrochemically dissolved into an electrolyte, leaving on the surface the noble element to form ligaments and pores with size of tens or hundreds of nanometers. The dealloying process can be performed from a crystalline or an amorphous precursor [2, 3] with a consequent change in the dealloying mechanism. This way, when amorphous precursors are used, ligaments result to be highly rough with nanometric holes that enable excellent optical and electrochemical properties. Nanoporous gold is produced in a large variety of morphologies and it can be modified by electrodeposition of Au nanoparticles on the ligament surface in order to enhance electrochemical or SERS (Surface Enhanced Raman Spectroscopy) activity.
In this work, the production of nanoporous gold via dealloying of Au-based amorphous precursors is outlined and compared with nanoporous gold produced from Fe-Au system, an innovative precursor that can allow a sensitive reduction in NPG production costs. For this purpose, Au-based amorphous alloys and Fe-Au crystalline alloys were chemically dealloyied in a solution of 2, 5, 10 and 14.4 M HNO₃ plus 0.5 M HF for different times. The surface morphology of de-alloyed samples was observed by scanning electron microscopy.
SERS measurements were performed on NPG samples synthesised following different routes in order to determine the morphology leading to the higher SERS activity. Bipyridine and melamine were used as probe molecules achieving detection limit down to 10-14 M. The enhancement of SERS effect on nanoporous gold is attributed to the localized enhanced electromagnetic fields around nanopores and to the electromagnetic coupling between ligaments. Ligaments constituted by several nanocrystals with random orientation and separated by grain boundaries leads to further enhancement of the SERS effect.

[1] J. Erlebacher, M.J. Aziz, a. Karma, N. Dimitrov, K. Sieradzki, Evolution of nanoporosity in dealloying, Nature 410 (2001) 450-453
[2] F. Scaglione, P. Rizzi, F. Celegato, L. Battezzati, Synthesis of nanoporous gold by free corrosion of an amorphous precursor, J. Alloys Compd. 615 (2014) S142-S147
[3] E.M. Paschalidou, F. Celegato, F. Scaglione, P. Rizzi, L. Battezzati, A. Gebert, S. Oswald, U. Wolff, L. Mihaylov, T. Spassov, The mechanism of generating nanoporous Au by de-alloying amorphous alloys, Acta Mater. 119 (2016) 177-183